Golf ball

ABSTRACT

A golf ball  2  includes a core  4  and a cover  6  that is composed of an inner cover  8  and an outer cover  10 . The core  4  has a radius of R (mm). A hardness Hs at a surface of the core  4  is greater than a hardness HL(R) when a linear approximation curve obtained by plotting a midpoint of each layer of the cover  6  in a thickness direction thereof in a graph in which a horizontal axis indicates a distance (mm) from a central point of the core  4  and a vertical axis indicates a hardness is represented by the following mathematical formula (I): 
         HL ( X )= A*X+B   (I),
 
     where X represents a distance (mm) from the central point of the core  4 , HL(X) represents a hardness, and A and B represent a gradient and an intercept, respectively, of the linear approximation curve.

This application claims priority on Patent Application No. 2012-006644 filed in JAPAN on Jan. 17, 2012. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls that include a core and a cover having two or more layers.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance. In particular, golf players place importance on flight performance upon a shot with a driver. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.

Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball easily curves. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.

Golf balls that include a core having excellent resilience performance are disclosed in JP61-37178, JP2008-212681 (US2008/0214324), JP2008-523952 (US2006/0135287 and US2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JP61-37178 is obtained from a rubber composition that includes a co-crosslinking agent and a crosslinking activator. This publication discloses palmitic acid, stearic acid, and myristic acid as the crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubber composition that includes an organic peroxide, a metal salt of an α,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubber composition that includes a metal salt of an unsaturated monocarboxylic acid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubber composition that includes a polybutadiene whose vinyl content is equal to or less than 2%, whose cis 1,4-bond content is equal to or greater than 80%, and which has an active end modified with an alkoxysilane compound.

An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of an outer—hard/inner—soft structure in a golf ball can achieve a low spin rate and a high launch angle. Modifications regarding a hardness distribution of a core are disclosed in JP6-154357 (U.S. Pat. No. 5,403,010), JP2008-194471 (US2008/0194358, US2008/0194359, US2008/0214325, and U.S. Pat. No. 7,344,455), and JP2008-194473 (US2008/0194357 and US2008/0312008).

In the core disclosed in JP6-154357, a JIS-C hardness H1 at the central point of the core is 58 to 73, a JIS-C hardness H2 in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core is equal to or greater than 65 but equal to or less than 75, a JIS-C hardness H3 at a point located at a distance of 15 mm from the central point is equal to or greater than 74 but equal to or less than 82, and a JIS-C hardness H4 at the surface of the core is equal to or greater than 76 but equal to or less than 84. The hardness H2 is greater than the hardness H1, the hardness H3 is greater than the hardness H2, and the hardness H4 is equal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at the central point of the core is equal to or greater than 30 but equal to or less than 48, a Shore D hardness at a point located at a distance of 4 mm from the central point is equal to or greater than 34 but equal to or less than 52, a Shore D hardness at a point located at a distance of 8 mm from the central point is equal to or greater than 40 but equal to or less than 58, a Shore D hardness at a point located at a distance of 12 mm from the central point is equal to or greater than 43 but equal to or less than 61, a Shore D hardness in a region that extends over a distance range from equal to or greater than 2 mm to equal to or less than 3 mm from the surface of the core is equal to or greater than 36 but equal to or less than 54, and a Shore D hardness at the surface of the core is equal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at the central point of the core is equal to or greater than 25 but equal to or less than 45, a Shore D hardness in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point is equal to or greater than 39 but equal to or less than 58, a Shore D hardness at a point located at a distance of 15 mm from the central point is equal to or greater than 36 but equal to or less than 55, and a Shore D hardness at the surface of the core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover. In the core, the hardness gradually increases from the central point of the core to the surface of the core. The difference between a JIS-C hardness at the surface and a JIS-C hardness at the central point is equal to or greater than 15. The hardness of the cover is greater than the hardness of the mid layer, and the hardness of the mid layer is greater than the hardness of the envelope layer.

Golf players also place importance on feel at impact of golf balls. Golf players prefer soft feel at impact. There is room for improvement in feel at impact. Golf players also place importance on stability of a spin rate and stability of a flight distance upon a shot with a long iron. Golf players desire golf balls whose spin rates and flight distances are unlikely to be influenced by a hitting point on a club head and an effective loft angle. There is room for improvement in stability of a spin rate and stability of a flight distance. An object of the present invention is to provide a golf ball having excellent feel at impact and excellent stability of a spin rate and excellent stability of a flight distance.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core having a radius of R (mm) and a cover that is positioned outside the core and has two or more layers. A JIS-C hardness Hs at a surface of the core is greater than a hardness HL(R) when a linear approximation curve obtained by plotting a midpoint of each layer of the cover in a thickness direction thereof in a graph in which a horizontal axis indicates a distance (mm) from a central point of the core and a vertical axis indicates a JIS-C hardness is represented by the following mathematical formula (I):

HL(X)=A*X+B  (I),

where X represents a distance (mm) from the central point of the core, HL(X) represents a JIS-C hardness, and A and B represent a gradient and an intercept, respectively, of the linear approximation curve.

In the golf ball according to the present invention, the relationship between the hardness near the surface of the core and the hardness of the cover is appropriate. The golf ball has excellent feel at impact.

Preferably, a difference between the hardness Hs and the hardness HL(R) is equal to or greater than 1.0. Preferably, the gradient A is equal to or greater than 6.5.

Preferably, a ratio (A/a) is greater than 4.0 when a linear approximation curve obtained by plotting two points at distances of 5.0 mm and 2.5 mm from the surface of the core toward the central point of the core and a point on the surface of the core in a graph in which a horizontal axis indicates a distance (mm) from the central point of the core and a vertical axis indicates a JIS-C hardness is represented by the following mathematical formula (II):

HC(x)=a*x+b  (II),

where x represents a distance (mm) from the central point of the core, HC(x) represents a JIS-C hardness, and a and b represent a gradient and an intercept, respectively, of the linear approximation curve.

Preferably, a JIS-C hardness of an outermost layer of the cover is greater than JIS-C hardnesses of any other parts of the golf ball. Preferably, a JIS-C hardness of an outermost layer of the cover is equal to or greater than 85 but equal to or less than 95. Preferably, a thickness of the cover is equal to or less than 2.5 mm.

The core can be formed by a rubber composition being crosslinked. Preferably, the rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) a carboxylate.

The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

Preferably, the rubber composition includes 1 parts by weight or greater but less than 40 parts by weight of the carboxylate (d) per 100 parts by weight of the base rubber (a). Preferably, the carboxylate (d) is a fatty acid salt. Preferably, a carbon number of a carboxylic acid component of the carboxylate (d) is equal to or greater than 4 but equal to or less than 30.

Preferably, the rubber composition further includes an organic sulfur compound (e). Preferably, the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols, thiurams, and metal salts thereof.

When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition further includes a metal compound (f).

Preferably, the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a). Preferably, the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a). Preferably, the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).

Preferably, a difference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at the central point of the core is equal to or greater than 15. Preferably, a difference (Ho−Hi) between a JIS-C hardness Ho of an outermost layer and a JIS-C hardness Hi of an innermost layer of the cover is equal to or greater than 5 but equal to or less than 30.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is a graph showing a hardness of a core of the golf ball in FIG. 1; and

FIG. 3 is a graph showing a hardness of a cover of the golf ball in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based on preferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6 positioned outside the core 4. The cover 6 is composed of an inner cover 8 and an outer cover 10. The cover 6 has a two-layer structure. On the surface of the outer cover 10, a large number of dimples 12 are formed. Of the surface of the golf ball 2, a part other than the dimples 12 is a land 14. The golf ball 2 includes a paint layer and a mark layer on the external side of the outer cover 10, but these layers are not shown in the drawing.

In the present invention, the “core 4” means a spherical part formed by crosslinking a rubber composition. In the present invention, the “cover 6” means a part that is positioned outside the core 4 and formed from a resin composition.

The golf ball 2 preferably has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably equal to or less than 44 mm and particularly preferably equal to or less than 42.80 mm. The golf ball 2 preferably has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

In the present invention, JIS-C hardnesses are measured at the following measuring points. The distances from the central point of the core 4 to these measuring points are as follows.

First point: 0.0 mm (central point)

Second point: 2.5 mm

Third point: 5.0 mm

Fourth point: 7.5 mm

Fifth point: 10.0 mm

Sixth point: 12.5 mm

Seventh point: 15.0 mm

Eighth point: 17.5 mm

Ninth point: surface

The hardnesses at the first to eighth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 4 that has been cut into two halves. The hardness at the ninth point is measured by pressing the JIS-C type hardness scale against the surface of the core 4. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 4 is L mm is represented by H(L). The hardness at the central point of the core 4 is represented by H(0). The surface hardness of the core 4 is represented by Hs.

In the present invention, a JIS-C hardness at a measuring point that is an internal point within the core 4 and whose distance from the surface of the core 4 is L mm is represented by Hs(−L). The distance L is measured along a line extending from the surface of the core 4 toward the central point of the core 4. A hardness at the point at a distance of 2.5 mm from the surface of the core 4 is represented by Hs(−2.5), and a hardness at the point at a distance of 5.0 mm from the surface of the core 4 is represented by Hs(−5.0). Hs(−5.0) and Hs(−2.5) are measured by pressing a JIS-C type hardness scale against a cut plane of the core 4 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. In the present embodiment, Hs(−5.0) is 75.6, Hs(−2.5) is 79.2, and Hs is 83.4.

FIG. 2 is a scatter graph showing a portion of a hardness distribution of the core 4 of the golf ball 2 in FIG. 1. The horizontal axis of the graph indicates a distance (mm) from the central point of the core 4. The vertical axis of the graph indicates a JIS-C hardness. In the graph, three points, namely, points whose distances L from the surface of the core 4 are 5.0 mm and 2.5 mm and a point on the surface of the core 4, are plotted.

FIG. 2 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the three measuring points. The linear approximation curve is represented by the following mathematical formula (II).

HC(x)=a*x+b  (II)

In this mathematical formula, x represents a distance (mm) from the central point of the core 4, HC(X) represents a JIS-C hardness, and a and b represent a gradient and an intercept, respectively, of the linear approximation curve. In the present embodiment, the gradient a is 1.56, and the intercept b is 52.40.

As described above, the cover 6 is composed of the inner cover 8 and the outer cover 10. In other words, the innermost layer of the cover 6 is the inner cover 8. A JIS-C hardness of the innermost layer is represented by Hi. The hardness Hi is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as a resin composition of the inner cover 8 is used.

The outermost layer of the cover 6 is the outer cover 10. A JIS-C hardness of the outermost layer is represented by Ho. The hardness Ho is measured by the same method as that for the hardness Hi. For the measurement, a slab formed from the same resin composition as a resin composition of the outer cover 10 is used.

FIG. 3 is a scatter graph showing a portion of a hardness distribution of the cover 6 of the golf ball 2 in FIG. 1. The horizontal axis of the graph indicates a distance (mm) from the central point of the core 4. The vertical axis of the graph indicates a JIS-C hardness. As described above, the cover 6 has a two-layer structure. Therefore, in the graph of FIG. 3, two points, namely, a point in the inner cover 8 and a point in the outer cover 10, are plotted. The distance from the central point of the core 4 to each layer of the cover 6 is the distance between the central point of the core 4 and the midpoint of each layer in the thickness direction thereof.

FIG. 3 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the two measuring points. The linear approximation curve is represented by the following mathematical formula (I).

HL(X)=A*X+B  (I)

In this mathematical formula, X represents a distance (mm) from the central point of the core 4, HL(X) represents a JIS-C hardness, and A and B represent a gradient and an intercept, respectively, of the linear approximation curve. In the present embodiment, the gradient A is 8.75, and the intercept B is −91.31.

In the golf ball 2 in which the cover 6 has three layers, a linear approximation curve is obtained on the basis of three measuring points. In the golf ball 2 in which the cover 6 has four layers, a linear approximation curve is obtained on the basis of four measuring points. In the golf ball 2 in which the cover 6 has five layers, a linear approximation curve is obtained on the basis of five measuring points. In the golf ball 2 in which the cover 6 has six layers, a linear approximation curve is obtained on the basis of six measuring points.

The JIS-C hardness Hs at the surface of the core 4 is greater than a hardness HL(R). Here, R represents the radius (mm) of core 4. Therefore, the hardness HL(R) is an assumed hardness on the linear approximation curve represented by the above mathematical formula (I). The golf ball 2 in which the hardness Hs is greater than the hardness HL(R) has excellent feel at impact. In the golf ball 2, soft feel at impact is obtained. The reason is inferred to be that shocks are absorbed at the boundary between the core 4 and the cover 6. The golf ball 2 further has excellent stability of a spin rate and excellent stability of a flight distance upon a shot with a long iron.

In light of feel at impact and stability of a spin rate and stability of a flight distance, the difference (Hs−HL(R)) between the hardness Hs and the hardness HL(R) is preferably equal to or greater than 1.0 and particularly preferably equal to or greater than 1.9. In light of ease of producing the golf ball 2, the difference (Hs−HL(R)) is preferably equal to or less than 40.

The ratio of the gradient A of the linear approximation curve represented by the above mathematical formula (I) to the gradient a of the linear approximation curve represented by the above mathematical formula (II) is preferably greater than 4.0. In other words, it is preferred that the gradient A is sufficiently greater than the gradient a. The golf ball 2 in which the ratio (A/a) is greater than 4.0 has excellent feel at impact. In the golf ball 2, soft feel at impact is obtained. The reason is inferred to be that shocks are absorbed by the inner cover 8. The golf ball 2 further has excellent stability of a spin rate and excellent stability of a flight distance upon a shot with a long iron.

In light of feel at impact and stability of a spin rate and stability of a flight distance, the ratio (A/a) is more preferably equal to or greater than 4.3 and particularly preferably equal to or greater than 4.8. In light of ease of producing the golf ball 2, the ratio (A/a) is preferably equal to or less than 20.

In light of feel at impact and stability of a spin rate and stability of a flight distance, the gradient A is preferably equal to or greater than 6.5. In light of ease of producing the cover 6, the gradient A is preferably equal to or less than 30.

The reason why the three points, the points at distances of 5.0 mm and 2.5 mm from the surface of the core 4 toward the central point of the core 4 and the point on the surface of the core 4, are selected in the calculation of the linear approximation curve (II) is that the hardness distribution of the cover 6 and a hardness distribution of a portion of the core 4 that is close to the cover 6 are compared to each other.

The core 4 is formed by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) a carboxylate.

Examples of the base rubber (a) include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% by weight and more preferably equal to or greater than 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal to or less than 2.0% by weight is preferred. The polybutadiene can contribute to the resilience performance of the golf ball 2. In this respect, the proportion of 1,2-vinyl bonds is preferably equal to or less than 1.7% by weight and particularly preferably equal to or less than 1.5% by weight.

From the standpoint that a polybutadiene having a low proportion of 1,2-vinyl bonds and excellent polymerization activity is obtained, a polybutadiene synthesized with a rare-earth-element-containing catalyst is preferred. In particular, a polybutadiene synthesized with a neodymium-containing catalyst, which is a lanthanum-series rare earth element compound, is preferred.

Examples of a preferable co-crosslinking agent (b) include:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylic acid (b1) or only the metal salt of the α,β-unsaturated carboxylic acid (b2) as the co-crosslinking agent (b). The rubber composition may include both the α,β-unsaturated carboxylic acid (b1) and the metal salt of the α,β-unsaturated carboxylic acid (b2) as the co-crosslinking agent (b).

The metal salt of the α,β-unsaturated carboxylic acid (b2) graft-polymerizes with the molecular chain of the base rubber, thereby crosslinking the rubber molecules. When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber.

Examples of the metal compound (f) include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. A compound that includes a bivalent metal is preferred. The compound that includes the bivalent metal reacts with the co-crosslinking agent (b) to form metal crosslinks. The metal compound (f) is particularly preferably a zinc compound. Two or more metal compounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examples of the metal component in the metal salt of the α,β-unsaturated carboxylic acid (b2) include sodium ion, potassium ion, lithium ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminum ion, tin ion, and zirconium ion. The metal salt of the α,β-unsaturated carboxylic acid (b2) may include two or more types of ions. From the standpoint that metal crosslinks are likely to occur between the rubber molecules, bivalent metal ions such as magnesium ion, calcium ion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt of the α,β-unsaturated carboxylic acid (b2) is particularly preferably zinc acrylate.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The crosslinking initiator (c) is preferably an organic peroxide. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 2, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

In the present invention, the co-crosslinking agent (b) is not included in the concept of the carboxylate (d). The carboxylic acid component of the carboxylate (d) has a carboxyl group. As described later, the carboxylic acid component exchanges a cationic component with the co-crosslinking agent (b).

The carbon number of the carboxylic acid component of the carboxylate (d) is preferably equal to or greater than 4 but equal to or less than 30. Examples of the carboxylate (d) include salts of aliphatic carboxylic acids (fatty acid salts) and salts of aromatic carboxylic acids. Fatty acid salts are preferred. The rubber composition may include a salt of a saturated fatty acid or may include a salt of an unsaturated fatty acid. A salt of a saturated fatty acid is preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylic acid (octanoic acid) (C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), lauric acid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid (C18), linolic acid (C18), linolenic acid (C18), 12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanic acid (C28), and melissic acid (C30). Two or more fatty acids may be used in combination.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group. Examples of aromatic carboxylic acids include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid (benzene-1,2,3-tricarboxylic acid), trimellitic acid (benzene-1,2,4-tricarboxylic acid), trimesic acid (benzene-1,3,5-tricarboxylic acid), mellophanic acid (benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid (benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzene hexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid), toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid (2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoic acid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid (2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoic acid), cuminic acid (4-isopropylbenzoic acid), uvitic acid (5-methylisophthalic acid), α-toluic acid (phenylacetic acid), hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid (3-phenylpropanoic acid).

The rubber composition may include an aromatic carboxylate substituted with a hydroxyl group, an alkoxy group, or an oxo group. Examples of this carboxylic acid can include salicylic acid (2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinic acid (hydroxy(methyl)benzoic acid), o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid (2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoic acid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), α-resorcylic acid (3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoic acid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid (3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxybenzoic acid), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoic acid), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid (2,4,5-trimethoxybenzoic acid), mandelic acid (hydroxy(phenyl)acetic acid), vanillylmandelic acid (hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid ((4-methoxyphenyl)acetic acid), homogentisic acid ((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid ((3,4-dihydroxyphenyl)acetic acid), homovanillic acid ((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid ((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid ((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid ((2,3-dimethoxyphenyl)acetic acid), homophthalic acid (2-(carboxymethyl)benzoic acid), homoisophthalic acid (3-(carboxymethyl)benzoic acid), homoterephthalic acid (4-(carboxymethyl)benzoic acid), phthalonic acid (2-(carboxycarbonyl)benzoic acid), isophthalonic acid (3-(carboxycarbonyl)benzoic acid), terephthalonic acid (4-(carboxycarbonyl)benzoic acid), benzilic acid (hydroxydiphenylacetic acid), atrolactic acid (2-hydroxy-2-phenylpropanoic acid), tropic acid (3-hydroxy-2-phenylpropanoic acid), melilotic acid (3-(2-hydroxyphenyl)propanoic acid), phloretic acid (3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid (3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid (3-(4-hydroxyphenyl)acrylic acid), umbellic acid (3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid (3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid (3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid (3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organic cation. Examples of the metal ion include sodium ion, potassium ion, lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion, aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two or more types of ions may be used in combination.

The organic cation is a cation having a carbon chain. Examples of the organic cation include organic ammonium ions. Examples of organic ammonium ions include primary ammonium ions such as stearylammonium ion, hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion; secondary ammonium ions such as dodecyl(lauryl)ammonium ion, and octadecyl(stearyl)ammonium ion; tertiary ammonium ions such as trioctylammonium ion; and quaternary ammonium ions such as dioctyldimethylammonium ion, and distearyldimethylammonium ion. Two or more types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, a magnesium salt, an aluminum salt, a zinc salt, an iron salt, a copper salt, a nickel salt, or a cobalt salt of octanoic acid, myristic acid, palmitic acid, stearic acid, or oleic acid. Particularly preferable carboxylates are zinc stearate and zinc octoate.

In light of linearity of the hardness distribution of the core 4, the amount of the carboxylate (d) is preferably equal to or greater than 1.0 parts by weight, more preferably equal to or greater than 2.0 parts by weight, and particularly preferably equal to or greater than 3.0 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 40 parts by weight, more preferably equal to or less than 20 parts by weight, and particularly preferably equal to or less than 10 parts by weight, per 100 parts by weight of the base rubber.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with zinc stearate for the purpose of improving dispersibility to rubber is present. When the rubber composition includes this zinc acrylate, the zinc stearate serves as the carboxylate (d). For example, when the rubber composition includes 25 parts by weight of zinc acrylate that includes 10% by weight of zinc stearate, the amount of the zinc stearate is regarded as 2.5 parts by weight, and the amount of the zinc acrylate is regarded as 22.5 parts by weight.

The rubber composition preferably further includes an organic sulfur compound (e). The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the core 4; and the degree of the outer-hard/inner-soft structure. An example of the organic sulfur compound (e) is an organic compound having a thiol group or a polysulfide linkage having 2 to 4 sulfur atoms. A metal salt of this organic compound is also included in the organic sulfur compound (e). Examples of the organic sulfur compound (e) include aliphatic compounds such as aliphatic thiols, aliphatic thiocarboxylic acids, aliphatic dithiocarboxylic acids, and aliphatic polysulfides; heterocyclic compounds; alicyclic compounds such as alicyclic thiols, alicyclic thiocarboxylic acids, alicyclic dithiocarboxylic acids, and alicyclic polysulfides; and aromatic compounds. Specific examples of the organic sulfur compound (e) include thiophenols, thionaphthols, polysulfides, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams, dithiocarbamates, and thiazoles. Preferable organic sulfur compounds (e) are thiophenols, thionaphthols, polysulfides having 2 to 4 sulfur atoms, thiurams, and metal salts thereof.

Particularly preferable organic sulfur compounds (e) from the standpoint that an outer-hard/inner-soft structure is easily obtained are 2-thionaphthol, bis(pentabromophenyl)disulfide, and 2,6-dichlorothiophenol.

From the standpoint that an outer-hard/inner-soft structure is easily obtained, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight, more preferably equal to or greater than 0.1 parts by weight, and particularly preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight, more preferably equal to or less than 3.0 parts by weight, and particularly preferably equal to or less than 1.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the core 4. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the core 4. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

During heating of the core 4, the heat of a crosslinking reaction of the base rubber remains near the central point of the core 4. Thus, during heating of the core 4, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. The carboxylate (d) reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. This exchange reaction is likely to take place in the central portion of the core 4 where the temperature is high, and is unlikely to take place near the surface of the core 4. In other words, breaking of metal crosslinks is likely to occur near the central portion of the core 4 and is unlikely to occur near the surface of the core 4. As a result, the crosslinking density of the core 4 increases from its inside toward its outside. In the core 4, the hardness linearly increases from its inside toward its outside. Furthermore, since the rubber composition includes the organic sulfur compound (e) together with the carboxylate (d), the gradient of the hardness distribution can be controlled, and the degree of the outer-hard/inner-soft structure of the core 4 can be increased.

In the core 4, the hardness gradually increases from its central point toward its surface. The difference (Hs−H(0)) between the surface hardness Hs and the central hardness H(0) is preferably equal to or greater than 15. The core 4 in which the difference (Hs−H(0)) is equal to or greater than 15 has an outer-hard/inner-soft structure. When the golf ball 2 is hit with a driver, the recoil (torsional return) in the core 4 is great, and thus spin is suppressed. The core 4 contributes to the flight performance of the golf ball 2. In light of flight performance, the difference (Hs−H(0)) is more preferably equal to or greater than 20 and particularly preferably equal to or greater than 28. From the standpoint that the core 4 can easily be formed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 4 is preferably equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 2 having a hardness H(0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 50.0. The core 4 having a hardness H(0) of 70.0 or less can achieve an outer-hard/inner-soft structure. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness H(0) is more preferably equal to or less than 65.0 and particularly preferably equal to or less than 60.0.

The hardness Hs at the surface of the core 4 is preferably equal to or greater than 75.0 but equal to or less than 95.0. The core 4 having a hardness Hs of 75.0 or greater can achieve an outer-hard/inner-soft structure. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 80.0 and particularly preferably equal to or greater than 82.0. The golf ball 2 having a hardness Hs of 96.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 90.0 and particularly preferably equal to or less than 85.0.

The core 4 preferably has a diameter of 38.0 mm or greater but 41.5 mm or less. The core 4 having a diameter of 38.0 mm or greater can achieve excellent resilience performance of the golf ball 2. In this respect, the diameter is more preferably equal to or greater than 38.5 mm and particularly preferably equal to or greater than 39.0 mm. In the golf ball 2 that includes the core 4 having a diameter of 41.5 mm or less, the inner cover 8 and the outer cover 10 can have sufficient thicknesses. The golf ball 2 that includes the inner cover 8 and the outer cover 10 which have large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 41.0 mm and particularly preferably equal to or less than 40.5 mm.

In light of feel at impact, the core 4 has an amount of compressive deformation (comp'n) of preferably 3.0 mm or greater and particularly preferably 3.5 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.5 mm and particularly preferably equal to or less than 4.0 mm.

For measurement of the amount of compressive deformation, a YAMADA type compression tester is used. In the tester, the core 4 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the core 4. The core 4, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the core 4 up to the state in which a final load of 1274 N is applied thereto, is measured.

For the inner cover 8, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, polystyrenes, polyesters, polyamides, and polyolefins.

Particularly preferable base polymers are ionomer resins. The golf ball 2 that includes the inner cover 8 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the inner cover 8. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 65% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more and 90% by weight or less of an α-olefin, and 10% by weight or more and 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more and 85% by weight or less of an α-olefin, 5% by weight or more and 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more and 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymers and the ternary copolymers, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. Particularly preferable ionomer resins are a copolymer formed with ethylene and acrylic acid and a copolymer formed with ethylene and methacrylic acid.

In the binary copolymers and the ternary copolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan AM7337”, “Himilan MK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the inner cover 8. An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

A preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer has excellent compatibility with ionomer resins. A resin composition including the styrene block-containing thermoplastic elastomer has excellent fluidity. The inner cover 8 including the styrene block-containing thermoplastic elastomer can achieve a great gradient A.

The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably equal to or greater than 10% by weight, more preferably equal to or greater than 12% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact of the golf ball 2, the content is preferably equal to or less than 50% by weight, more preferably equal to or less than 47% by weight, and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include alloys of olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, and hydrogenated products thereof. The olefin component in the alloy is presumed to contribute to improvement of compatibility with ionomer resins. Use of this alloy improves the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferably used. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “Rabalon T3221C”, “Rabalon T3339C”, “Rabalon SJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”, “Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “Rabalon SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the inner cover 8 in an adequate amount.

In light of flight performance, the hardness Hi of the inner cover 8 is preferably equal to or greater than 70, more preferably equal to or greater than 73, and particularly preferably equal to or greater than 76. In light of feel at impact of the golf ball 2, the hardness Hi is preferably equal to or less than 95, more preferably equal to or less than 90, and particularly preferably equal to or less than 87.

In light of feel at impact, the inner cover 8 has a thickness of preferably 0.5 mm or greater and particularly preferably 0.8 mm or greater. In light of flight performance, the thickness of the inner cover 8 is preferably equal to or less than 2.0 mm and particularly preferably equal to or less than 1.5 mm.

For the outer cover 10, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The ionomer resin described above for the inner cover 8 can also be used for the outer cover 10. The golf ball 2 that includes the outer cover 10 including the ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the outer cover 10. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 70% by weight, more preferably equal to or greater than 80% by weight, and particularly preferably equal to or greater than 90% by weight.

Examples of a resin that can be used in combination with an ionomer resin include polyesters, polyamides, polyolefins, polystyrenes, and polyurethanes.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the outer cover 10 in an adequate amount.

From the standpoint that a great gradient A is obtained, the hardness Ho of the outer cover 10 is preferably equal to or greater than 85, more preferably equal to or greater than 90, and particularly preferably equal to or greater than 92. In light of feel at impact, the hardness Ho is preferably equal to or less than 95. In light of suppression of spin, it is preferred that the JIS-C hardness of the outermost layer (outer cover 10) of the cover 6 is greater than the JIS-C hardnesses of any other parts of the golf ball 2.

In light of feel at impact, the outer cover 10 has a thickness of preferably 0.4 mm or greater and particularly preferably 0.6 mm or greater. In light of flight performance, the thickness of the outer cover 10 is preferably equal to or less than 2.0 mm and particularly preferably equal to or less than 1.5 mm.

For forming the outer cover 10, known methods such as injection molding, compression molding, and the like can be used. When forming the outer cover 10, the dimples 12 are formed by pimples on the cavity face of a mold.

The difference (Ho−Hi) between the hardness Ho of the outermost layer (outer cover 10) and the hardness Hi of the innermost layer (inner cover 8) of the cover 6 is preferably equal to or greater than 5. The golf ball 2 in which the difference (Ho−Hi) is equal to or greater than 5 has excellent feel at impact. In this respect, the difference (Ho−Hi) is more preferably equal to or greater than 7 and particularly preferably equal to or greater than 9. In light of ease of producing the golf ball 2, the difference (Ho−Hi) is preferably equal to or less than 30.

The cover 6 has a thickness of preferably 2.5 mm or less. In the golf ball 2 that includes the cover 6 having a thickness of 2.5 mm or less, the core 4 is sufficiently large. The core 4 can contribute to the resilience performance of the golf ball 2. In this respect, the thickness is more preferably equal to or less than 2.2 mm and particularly preferably equal to or less than 2.0 mm. In light of ease of forming the cover 6, the thickness is more preferably equal to or greater than 0.6 mm and particularly preferably equal to or greater than 1.0 mm. The thickness of the cover 6 is the sum of the thicknesses of all the layers of the cover 6.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 26 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-thionaphthol, 10 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a core with a diameter of 39.5 mm.

A resin composition was obtained by kneading 40 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 20 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold including upper and lower mold halves each having a hemispherical cavity. The resin composition was injected around the core by injection molding to form an inner cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329”), 30 parts by weight of an ethylene-methacrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide (A220), and 0.2 parts by weight of a hindered amine light stabilizer (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core and the inner cover was placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and that has a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form an outer cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the outer cover. A clear paint including a two-component curing type polyurethane as a base material was applied to the outer cover to obtain a golf ball of Example 1 with a diameter of 42.7 mm.

Examples 2 to 8 and Comparative Examples 1 to 4

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 4 were obtained in the same manner as Example 1, except the specifications of the core and the inner cover were as shown in Tables 3 and 4 below. The composition of the core of each golf ball is the same as the composition of the core of Example 1. A hardness distribution of the core is shown in Table 1 below. The composition of the inner cover is shown in Table 2 below.

TABLE 1 Hardness of Core (JIS-C) Ex. 2-5 Ex. 1 Comp. Ex. Ex. Ex. 8 Comp. Ex. 1 2-3 6-7 Comp. Ex. 4 H(0.0) 54.0 54.0 54.0 54.0 H(2.5) 59.8 59.8 59.8 59.8 H(5.0) 63.0 63.0 63.0 63.0 H(7.5) 64.6 64.6 64.6 64.6 H(10.0) 67.0 67.0 67.0 67.0 H(12.5) 71.8 71.8 71.8 71.8 H(15.0) 76.0 76.0 76.0 76.0 H(17.5) 79.5 79.5 79.5 79.5 Hs 83.4 83.0 82.5 82.1

TABLE 2 Composition of Inner Cover (parts by weight) i ii iii iv v vi Himilan AM7337 51 45 42 40 24 26 Himilan AM7329 40 40 40 40 50 40 Rabalon T3221C 9 15 18 20 26 34 Titanium 6 6 6 6 6 6 dioxide Hardness 89.0 87.0 86.0 85.0 83.0 76.0 (JIS-C)

[Feel at Impact]

Ten golf players hit golf balls with sand wedges and were asked about feel at impact. Each golf player evaluated the feel at impact on a five-point scale. The average is shown in Tables 3 and 4 below. A higher value indicates a better result.

[Flight Test]

A 5-iron (trade name “NEW XXIO”, manufactured by SR1 Sports Limited, shaft hardness: R) was attached to a swing machine manufactured by True Temper Co. A golf ball was hit under the following condition 1, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S1 of spin rates and the average value L1 of flight distances obtained by 10 measurements were calculated. Furthermore, a golf ball was hit under the following condition 2, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S2 of spin rates and the average value L2 of flight distances obtained by 10 measurements were calculated.

Condition 1

-   -   Head speed: 34 m/sec     -   Effective loft angle: 20.5°     -   Hitting point: a point higher than the face center by 5 mm.

Condition 2

-   -   Head speed: 34 m/sec     -   Effective loft angle: 23.5°     -   Hitting point: a point lower than the face center by 5 mm.         The spin rate under the condition 1 is lower than the spin rate         under the condition 2. The flight distance under the condition 1         is larger than the flight distance under the condition 2. The         difference (L1−L2) and the difference (S2−S1) are shown in         Tables 3 and 4 below.

TABLE 3 Results of Evaluation Comp. Ex. 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Core Hs − H(0) 29.4 29.4 29.0 29.0 29.0 29.0 Diameter (mm) 39.5 39.5 39.1 39.1 39.1 39.1 comp'n 3.85 3.85 3.85 3.85 3.85 3.85 Hs(−5.0) 75.6 75.6 75.2 75.2 75.2 75.2 Hs(−2.5) 79.2 79.2 78.9 78.9 78.9 78.9 Hs 83.4 83.4 83.0 83.0 83.0 83.0 Inner cover Composition iv ii vi v iv iii Thickness 0.8 0.8 1.0 1.0 1.0 1.0 (mm) Hardness Hi 85.0 87.0 76.0 83.0 85.0 86.0 Outer cover Thickness 0.8 0.8 0.8 0.8 0.8 0.8 (mm) Hardness Ho 92.0 92.0 92.0 92.0 92.0 92.0 Gradient a 1.56 1.56 1.55 1.55 1.55 1.55 Intercept b 52.40 52.40 52.59 52.59 52.59 52.59 Gradient A 8.75 6.25 17.78 10.00 7.78 6.67 Intercept B −91.31 −38.94 −280.44 −117.50 −70.94 −47.67 HL(R) 81.5 84.5 67.1 78.0 81.1 82.7 Hs − HL(R) 1.9 −1.1 15.9 5.0 1.9 0.3 A/a 5.6 4.0 11.5 6.5 5.0 4.3 Ho − Hi 7.0 5.0 16.0 9.0 7.0 5.0 Feel at impact 4.2 2.5 4.8 4.5 4.0 3.6 L1 − L2 (m) 9.6 11.3 6.9 8.8 10.4 11.1 S1 (rpm) 3355 3151 3923 3685 3292 3158 S2 (rpm) 4530 4486 4711 4596 4503 4476 S2 − S1 (rpm) 1175 1335 788 911 1211 1318

TABLE 4 Results of Evaluation Comp. Comp. Comp. Ex. 2 Ex. 3 Ex. 6 Ex. 7 Ex. 8 Ex. 4 Core Hs − H(0) 29.0 29.0 28.5 28.5 28.1 28.1 Diameter (mm) 39.1 39.1 38.5 38.5 37.9 37.9 comp'n 3.85 3.85 3.85 3.85 3.85 3.85 Hs(−5.0) 75.2 75.2 74.7 74.7 74.2 74.2 Hs(−2.5) 78.9 78.9 78.5 78.5 78.0 78.0 Hs 83.0 83.0 82.5 82.5 82.1 82.1 Inner cover Composition ii i v iv v iv Thickness 1.0 1.0 1.0 1.0 1.0 1.0 (mm) Hardness Hi 87.0 89.0 83.0 85.0 83.0 85.0 Outer cover Thickness 0.8 0.8 1.1 1.1 1.4 1.4 (mm) Hardness Ho 92.0 92.0 92.0 92.0 92.0 92.0 Gradient a 1.55 1.55 1.55 1.55 1.57 1.57 Intercept b 52.59 52.59 52.57 52.57 52.25 52.25 Gradient A 5.56 3.33 8.57 6.67 7.50 5.83 Intercept B −24.39 22.17 −86.29 −46.67 −62.88 −28.46 HL(R) 84.2 87.3 78.7 81.7 79.3 82.1 Hs − HL(R) −1.2 −4.3 3.8 0.8 2.8 0.0 A/a 3.6 2.2 5.5 4.3 4.8 3.7 Ho − Hi 5.0 3.0 9.0 7.0 9.0 7.0 Feel at impact 2.4 1.5 3.9 3.3 3.6 2.8 L1 − L2 (m) 11.6 12.1 9.1 10.9 10.2 12.2 S1 (rpm) 3098 3046 3507 3181 3314 3040 S2 (rpm) 4466 4447 4533 4477 4498 4468 S2 − S1 (rpm) 1368 1401 1026 1296 1184 1428

As shown in Tables 3 and 4, the golf ball according to each Example has excellent feel at impact. Furthermore, the difference (S2−S1) of the golf ball according to each Example is equal to or less than 1318 rpm. In other words, in the golf ball according to the present invention, the spin rate is sufficiently stabilized. From these results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playing golf on golf courses and practicing at driving ranges. The above descriptions are merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core having a radius of R (mm) and a cover that is positioned outside the core and has two or more layers, wherein a JIS-C hardness Hs at a surface of the core is greater than a hardness HL(R) when a linear approximation curve obtained by plotting a midpoint of each layer of the cover in a thickness direction thereof in a graph in which a horizontal axis indicates a distance (mm) from a central point of the core and a vertical axis indicates a JIS-C hardness is represented by the following mathematical formula (I): HL(X)=A*X+B  (I), where X represents a distance (mm) from the central point of the core, HL(X) represents a JIS-C hardness, and A and B represent a gradient and an intercept, respectively, of the linear approximation curve.
 2. The golf ball according to claim 1, wherein a difference between the hardness Hs and the hardness HL(R) is equal to or greater than 1.0.
 3. The golf ball according to claim 1, wherein the gradient A is equal to or greater than 6.5.
 4. The golf ball according to claim 1, wherein a ratio (A/a) is greater than 4.0 when a linear approximation curve obtained by plotting two points at distances of 5.0 mm and 2.5 mm from the surface of the core toward the central point of the core and a point on the surface of the core in a graph in which a horizontal axis indicates a distance (mm) from the central point of the core and a vertical axis indicates a JIS-C hardness is represented by the following mathematical formula (II): HC(x)=a*x+b  (II), where x represents a distance (mm) from the central point of the core, HC(x) represents a JIS-C hardness, and a and b represent a gradient and an intercept, respectively, of the linear approximation curve.
 5. The golf ball according to claim 1, wherein a JIS-C hardness of an outermost layer of the cover is greater than JIS-C hardnesses of any other parts of the golf ball.
 6. The golf ball according to claim 1, wherein a JIS-C hardness of an outermost layer of the cover is equal to or greater than 85 but equal to or less than
 95. 7. The golf ball according to claim 1, wherein a thickness of the cover is equal to or less than 2.5 mm.
 8. The golf ball according to claim 1, wherein the core is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) a carboxylate, and the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 9. The golf ball according to claim 8, wherein the rubber composition includes 1 parts by weight or greater but less than 40 parts by weight of the carboxylate (d) per 100 parts by weight of the base rubber (a).
 10. The golf ball according to claim 8, wherein the carboxylate (d) is a fatty acid salt.
 11. The golf ball according to claim 8, wherein a carbon number of a carboxylic acid component of the carboxylate (d) is equal to or greater than 4 but equal to or less than
 30. 12. The golf ball according to claim 8, wherein the rubber composition further includes an organic sulfur compound (e).
 13. The golf ball according to claim 12, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols, thiurams, and metal salts thereof.
 14. The golf ball according to claim 8, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
 15. The golf ball according to claim 8, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 16. The golf ball according to claim 8, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 17. The golf ball according to claim 12, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 18. The golf ball according to claim 1, wherein a difference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at the central point of the core is equal to or greater than
 15. 19. The golf ball according to claim 1, wherein a difference (Ho−Hi) between a JIS-C hardness Ho of an outermost layer and a JIS-C hardness Hi of an innermost layer of the cover is equal to or greater than 5 but equal to or less than
 30. 